Search

In recent years, the Industry 4.0 paradigm has gained special relevance, focused on the integration of smart structures to increase the efficiency of processes. Photonics, with laser advanced manufacture and light-based sensors, can offer great possibilities in this field. Within advanced manufacturing, femtosecond lasers allow inscribing different optical structures. Waveguides are one of the most widely used structures in photonic sensors. This work proposes the design, simulation and manufacture of symmetric cladding waveguides in optical fibers using the slit beam shaping technique. Cladding waveguides characterized by their low losses (0.21 dB/cm), quasi?step-index refractive index profile, and circular cross-section are used to couple evanescent light from the core of a single-mode fiber to each of the four cores of a multicore fiber. This work has been supported by the Ministerio de Ciencia, Innovación y Universidades and Agencia Estatal de Investigación (PID2019-107270RB- C21), the Ministerio de Educación, Cultura y Deporte of Spain (PhD grant FPU2018/02797), and FEDER funds.

In this work, a novel technique to create positive-negative tunable liquid crystal lenses is proposed and experimentally demonstrated. This structure is based on two main elements, a transmission line acting as a voltage divider and concentric electrodes that distribute the voltage homogeneously across the active area. This proposal avoids all disadvantages of previous techniques, involving much simpler fabrication process (a single lithographic step) and voltage control (one or two sources). In addition, low voltage signals are required. Lenses with switchable positive and negative focal lengths and a simple, low voltage control are demonstrated. Moreover, by using this technique other optical devices could be engineered, e.g. axicons, Powell lenses, cylindrical lenses, Fresnel lenses, beam steerers, optical vortex generators, etc. For this reason, the proposed technique could open new venues of research in optical phase modulation based on liquid crystal materials. This work was supported by Comunidad de Madrid and FEDER Program (S2018/NMT-4326), the Ministerio de Economía y Competitividad of Spain (TEC2016-77242-C3-1-R and TEC2016-76021-C2-2-R), the FEDER/ Ministerio de Ciencia, Innovación y Universidades and Agencia Estatal de Investigación (RTC2017-6321-1). Also, the Ministry of National Defense of Poland (GBMON/13-995/2018/WAT).

In this work, a novel dielectric metasurface consisting of square slotted arrays etched in a silicon layer is proposed and theoretically demonstrated. The structure is designed to support electromagnetically induced transparency (EIT) based on quasi-bound states in the continuum (qBIC). Specifically, the metasurface consists of square slots with a silicon gap that breaks the symmetry of the structure. Thanks to the interaction of the sharp quasi-BIC resonances with a broadband background mode, an extremely high Q factor EIT response of 6⋅106 is demonstrated (considering the length scales feasible during fabrication and optical losses). Moreover, the resonator possesses a simple bulk geometry and subwavelength dimensions. The proposed metasurface, its high Q factors, and strong energy confinement may open new avenues of research on light-matter interactions in emerging applications in non-linear devices, lasing, biological sensors, optical communications, etc. This work was supported by Comunidad de Madrid and FEDER Program (S2018/NMT-4326), the Ministerio de Economía y Competitividad of Spain (TEC2016-77242-C3-1-R and TEC2016-76021-C2-2-R), the FEDER/Ministerio de Ciencia, Innovación y Universidades and Agencia Estatal de Investigación (RTC2017-6321-1, PID2019-107270RB-C21 and PID2019-109072RB-C31).

In this paper, diffractive elements in one and two dimensions have been inscribed at the end-face of single and multicore optical fibers using femtosecond lasers. Different sensing applications have been also presented on the basis of their projected far-field patterns. The manufacturing of 2D structures was achieved using two approaches: line writing and its equivalent point-by-point technique in order to speed up the process. Specific properties of both the techniques have been exploited by inscribing 2-direction and circular gratings. When diffraction grating is inscribed at the end-face of a four core fiber, the 2D interference pattern is transferred into diffraction orders with the possibility of decoupling the interferometric contribution of selected core. Particularly, with a circular diffraction grating, the multicore far-field pattern exhibits two perpendicular and independent interference patterns, as only opposite cores are interfering with each other. These patterns exhibit period and, hence, sensitivity with phase perturbation, proportional to diffraction order angle. The 2D diffraction grating inscribed in the single mode fiber has been employed for bidimensional displacement insensitive sensing, exhibiting linear response, while aforementioned structures written in multicore fiber have been proposed for bending vector sensing, thereby showing higher sensitivity than that of its pristine counterpart.

In this work, we propose and experimentally demonstrate a new distributed short linear cavity fiber laser. At one of the cavity ends, fabricated by a commercial femtosecond fiber laser chirped pulse amplfier, an artificially controlled backscattering erbium doped fiber section has been connected. This distributed reflector acts also as a saturable absorber, leading to the generation of tunable and switchable single longitudinal-mode laser emissions. The distributed reflector consists of 9 micro-drilled sections of about 1cm each one and randomly spread throughout 2 meters of highly doped erbium fiber. The total length of the fiber laser is 9.5 m and the laser shows a single mode behavior at all the emitted wavelengths. Using this new kind of reflecting saturable absorber, single and multiple single-mode emissions can be obtained. The achieved laser presents a pump threshold as low as 45 mW and shows up to 8 different single-mode emission lines with an optical signal to noise ratio of 45dB. This work was supported in part by the MINECO/AEI (Ministerio de Economía y Competitividad/Agencia Estatal de Investigación), Spain, under Grant TEC2016-76021-C2-1-R, Grant TEC2016-76021-C2-2-R, and Grant PID2019-107270RB; in part by the Ministerio de Educación, Cultura y Deporte of Spain under Grant FPU2018/02797, and in part by the Projects for Young Researches UPNA 2019 (Universidad Publica de Navarra) and FEDER Funds (European Regional Development Fund).

There is an increasing need to control light phase with tailored precision via simple means in both fundamental science and industry. One of the best candidates to achieve this goal are electro-optical materials. In this work, a novel technique to modulate the spatial phase profile of a propagating light beam by means of liquid crystals (LC), electro-optically addressed by indium-tin oxide (ITO) grating microstructures, is proposed and experimentally demonstrated. A planar LC cell is assembled between two perpendicularly placed ITO gratings based on microstructured electrodes. By properly selecting only four voltage sources, we modulate the LC-induced phase profile such that non-diffractive Bessel beams, laser stretching, beam steering, and 2D tunable diffraction gratings are generated. In such a way, the proposed LC-tunable component performs as an all-in-one device with unprecedented characteristics and multiple functionalities. The operation voltages are very low and the aperture is large. Moreover, the device operates with a very simple voltage control scheme and it is lightweight and compact. Apart from the demonstrated functionalities, the proposed technique could open further venues of research in optical phase spatial modulation formats based on electro-optical materials. This work was supported by the Comunidad de Madrid and FEDER Program (S2018/NMT-4326), the Ministerio de Economía y Competitividad of Spain (TEC2016-77242-C3-1-R and TEC2016-76021-C2-2-R), the FEDER/Ministerio de Ciencia, Innovación y Universidades and Agencia Estatal de Investigación (RTC2017-6321-1, PID2019-109072RB-C31 and PID2019-107270RB-C21). The authors also acknowledge the support by the Ministry of National Defense of Poland (GBMON/13-995/2018/WAT), Military University of Technology (Grant no. 23-895).

In this work, the development of plane-by-plane (Plb-Pl) fiber Bragg gratings (FBGs) by using a single pulse is reported. A slit oriented along the longitudinal axis of the fiber is employed for this. The purpose is to shape the beam so that the focal volume is spatially wider in the transverse direction of the fiber. In this way, it is possible to make 2-D modifications of the refractive index whose width and height depends on the slit width and the pulse energy, respectively. Through an analytical mathematical modelling, the relationship between the diameter of the Gaussian laser beam (D ) and the slit width (s = D,) is determined in order to obtain a circular cross-section. In the experimental results carried out, it can be observed that D /D, = 3 ratio cause positive and uniform refractive index changes, compared to the negative and inhomogeneous refractive index changes corresponding to point-by-point (PbP) FBGs. Likewise, Pl-b-Pl FBGs, presenting a broader modification of the core cross-section, show significantly lower losses in transmission (0.3 dB), as well as better reflectivity and FWHM. It is observed that these three parameters (losses, reflectivity and FWHM) have a monotonous tendency according to the slit width. Polarization-dependence is also evaluated. This work was supported in part by the project TEC2016-76021-C2-2-R (AEI/FEDER) of the Spanish government and FEDER funds and in part by the Grant FPU2018/02797 from the Ministry of Science, Innovation and Universities of the Spanish government.

The consolidation of laser micro/nano processing technologies has led to a continuous increase in the complexity of optical fiber sensors. This new avenue offers novel possibilities for advanced sensing in a wide set of application sectors and, especially in the industrial and medical fields. In this review, the most important transducing structures carried out by laser processing in optical fiber are shown. The work covers different types of fiber Bragg gratings with an emphasis in the direct-write technique and their most interesting inscription configurations. Along with gratings, cladding waveguide structures in optical fibers have reached notable importance in the development of new optical fiber transducers. That is why a detailed study is made of the different laser inscription configurations that can be adopted, as well as their current applications. Microcavities manufactured in optical fibers can be used as both optical transducer and hybrid structure to reach advanced soft-matter optical sensing approaches based on optofluidic concepts. These in-fiber cavities manufactured by femtosecond laser irradiation followed by chemical etching are promising tools for biophotonic devices. Finally, the enhanced Rayleigh backscattering fibers by femtosecond laser dots inscription are also discussed, as a consequence of the new sensing possibilities they enable This research was funded by the Ministerio de Economía y Competitividad of Spain (TEC2016-76021-C2-2-R), the FEDER/Ministerio de Ciencia, Innovación y Universidades and Agencia Estatal de Investigación (PID2019- 107270RB-C21), and the Ministerio de Educación, Cultura y Deporte of Spain (PhD grant FPU2018/02797).

We demonstrate a technique to engineer cylindrical and Powell liquid crystal lenses with positive or negative optical power. The device is based on two indium-tin-oxide electrode combs and a microstructured voltage transmission electrode. The technique features the advantages of a multielectrode lens, albeit using a single lithographic step and only two voltage sources. Extensive control of the phase profile across the device active area is demonstrated, achieving both positive and negative optical power. The lens aperture is not constrained by the geometrical parameters and can be scaled to larger values. This work was supported in part by the Comunidad de Madrid and FEDER Program under Grant S2018/NMT-4326, in part by the Ministerio de Economía y Competitividad of Spain under Grant TEC2013-47342-C2-2-R and Grant TEC2016-76021-C2-2-R, in part by the FEDER/Ministerio de Ciencia, Innovación y Universidades and Agencia Estatal de Investigación under Grant RTC2017-6321-1, Grant PID2019-109072RB-C31, and Grant PID2019-107270RB-C21, in part by the Ministry of National Defense of Poland under Grant GBMON/13-995/2018/WAT, and in part by the Military University of Technology under Grant 23-895.

In this Letter, we present an analysis of the backscattering generated in a microdrilled tapered fiber within a short-linear-cavity fiber laser. We propose and experimentally demonstrate a fiber cavity laser based on a tapered optical fiber segment that generates artificially controlled backscattering. The 50-mu m-waist taper was fabricated from a standard single-mode fiber, and the taper was subsequently microdrilled using a femtosecond laser micromachining system. The laser generation properties conferred by the microdrilled optical fiber taper were experimentally investigated. A single-wavelength laser emission at 1562 nm with an output power of 10 dBm and an optical signal-to-noise ratio greater than 45 dB is demonstrated. Ministerio de Economía y Competitividad (MINECO) (TEC 2016-76021-C2-R); European Regional Development Fund (ERDF).

In this work, we present an experimental measurement of temperature and strain sensitivities of a micro-drilled optical fiber (MDOF). The MDOF consisted of a quasi-randomly distributed reflector along a single mode fiber (SMF). A fiber cavity laser based on MDOF was experimentally studied, attaining a single-wavelength laser emission centered at 1568.6nm. The output power level obtained from this single-laser oscillation when pumped at 140mW was around - 9.6dBm, and an optical signal to noise ratio (OSNR) of around 45dB was measured. Although temperature sensitivities of fiber Bragg gratings used as sensors are similar to our MDOF, strain sensitivity is enhanced around one order of magnitude when the MDOF was used. This work was supported by the Spanish AEI TEC2016-76021-C2, FEDER Funds, and the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No 838143.

In this paper, a 6 mm hybrid Mach-Zehnder Interferometer (MZI) has been manufactured within a standard optical fiber using multiscan inscription with femtosecond laser. This technique allows the employ of cladding waveguides (CWG) as sensing arms for the interferometer. Refracted Near Field (RNF) profilometry and Quantitative Phase Microscopy (QPM) consistently suggest that CWG exhibit a smooth Type I refractive index change (RIC) that increases with the number of scans. This makes the scan number a potential way to control the coupling and Free Spectral Range (FSR) of the manufactured MZI. Its combination with a fiber Bragg grating (FBG) inscribed in the core makes possible to discriminate between different parameters. This work has been supported by the project TEC2016-76021-C2-2-R (AEI/FEDER) of the Spanish government and FEDER funds.

In this paper, a novel approach for hybrid systems combining Raman distributed sensors with fiber Bragg grating (FBG) sensors to carry out distributed and quasi-distributed temperature/strain measurements was proposed and experimentally demonstrated. Three FBGs were inscribed by the point-by-point technique in a simple setup for type I femtosecond inscription in a pure silica core multimode gold-coated fiber. Employing a single fiber, temperatures up to 600 °C and strains up to 4144 µE approximately were measured simultaneously and without interferences between both distributed and point measurements. Moreover, a new calibration technique was implemented to calibrate the distributed temperature system using the FBG measurements as reference. Spanish Government FEDER Funds (TEC2016-76021-C2-2-R).

In this work, a dual-wavelength fiber ring laser working on the SLM regime is proposed and experimentally verified. A proof-of-concept device has been tested employing high precision small-sized SLM fiber lasers to complement the feedback produced by a passive FBG. The ring laser output inherits the spectral benefits of its SLM seed lasers but maintaining some power-related properties of ring cavities. These spectral benefits can also be achieved for a single wavelength, simplifying the requirements regarding power equalization between different wavelengths to obtain the SLM regime. The experimental results exhibit a remarkably good wavelength stability and SLM operation of a fiber ring laser with several meters of cavity length, both with single and multi-wavelength configurations. This work has been supported by the project TEC2016-76021-C2-2-R of the Spanish Government cofunded by EU/FEDER funds. Special thanks to Fibercore for their support.

A feasibility study of the employment of a fiber laser working as the laser source of a Brillouin distributed sensor is presented in this paper. The erbium fiber ring laser structure designed works on the single longitudinal mode regime. The main parameters of the ring cavity are analyzed in terms of the laser requirements and the performance of a Brillouin optical time domain analysis sensor using the fiber laser as light source. This work was supported by the Spanish Government and FEDER funds under Project TEC2016-76021-C2-2-R

In this letter, an annealing process of UV-written fiber Bragg gratings (FBGs) has been employed to achieve FBGs with narrower reflection spectra to be employed in fiber lasers. These FBGs have been used to design distributed Bragg relfector (DBR) laser sensors working on the single longitudinal mode (SLM) regime even at high temperatures. A fiber laser sensor working on multimode regime has been written into commercial Er-doped fiber before annealing its mirror FBGs, removing the unstable UV-induced modulation part and narrowing the FBGs’ combined response. This process obtains a DBR fiber laser sensor working on SLM regime at high temperatures ( $T=500~^{\circ }\text{C}$ ), and also exhibiting a remarkably good linearity and stability. The achieved results confirm the empirically adjusted aging model and prove its usefulness to design high precision devices able to work at high temperatures.

A new optical fiber sensor for monitoring the compaction pressure of a composite material during the molding process is designed and experimentally demonstrated. The sensor is based on a Fiber Bragg Grating embedded into a composite material, and it is able to measure the compaction pressure during the manufacturing process and the strain information while the structure is in operation. The proposed design has been characterized before its embodiment, showing a logarithmic response to the pressure. The sensor also exhibits a linear response to both strain and temperature, making it suitable for structural health monitoring applications.

A hexagonal diffraction grating has been inscribed at the end of an optical fiber using a femtosecond laser. The achieved structure has been employed to measure the 2D tilt angle being also insensitive to displacement. The achieved results suggest the potential applications of these structures to many scenarios.

Tilted diffraction gratings have been inscribed by femtosecond laser in the bulk of optical fiber end-faces. One end-fiber diffraction grating has been applied for temperature sensing based on order displacement, achieving linear behavior up to 75°C.

Femtosecond laser written waveguides inside optical fiber are employed to manufacture in-fiber Mach-Zehnder Interferometers. Quantitative phase and birefringence measurements suggest light confinement below focal volume due to stress accumulation.

In this paper we present the employment of an Erbium Fiber Ring Laser structure working on the Single Longitudinal Mode regime within a Brillouin optical time domain analyzer. An analysis of some key laser parameters will be carried out, proving that a very stable operation is achieved. The associated performance of the BOTDA system, as well as the possible benefits derived from the use of the proposed laser design will also be discussed. This work has been supported by the projects TEC2013-47264-C2-1-R and TEC2016-76021-C2-2-R.

A distributed Bragg reflector fiber laser sensor based on a commercial high-birefringence Er-doped fiber has been proposed. The spectral overlap of two uniform fiber Bragg gratings has been employed as filtering technique to achieve a sensing head that operates in the single longitudinal mode regime. The high-birefringence Er-doped fiber has been employed to remove a polarization mode, only allowing the oscillation of a single polarization mode. In this way, a single frequency operation with a very stable laser output has been achieved. The proposed fiber laser exhibits a linear response for both strain and temperature variations, keeping the SLM regime and the polarization mode suppression.

This paper presents a new stable dual-wavelength fiber-ring laser based on erbium-doped fiber amplification. The wavelength selection is made by a combination of fiber Bragg gratings and phase shift fiber Bragg gratings. Two laser emission lines were obtained simultaneously in single-longitudinal-mode operation showing a power-instability lower than 0.35 dB, an OSNR higher than 55 dB, a narrow-linewidth of 13 KHz and an optical output power around 0 dBm for both wavelengths. This proposed configuration allows the individual control of the loss of each channel.

In this letter, different parameters of the spectral overlapping filtering technique have been analyzed for laser structures. Despite the promising preliminary experimental results obtained using this filtering technique, the optimization of the fiber Bragg grating (FBG) parameters to reduce the bandwidth of the achieved filter depending on the fiber laser requirements and configuration, should be performed. The simulated results have been confirmed during the experimental tests, where the same bandwidths and power evolutions have been measured using a ring laser configuration. This technique has been also tested for very narrow filtering purposes by achieving the single longitudinal mode operation with a ring cavity of L ≈ 8 m, using only two uniform FBGs.

A technique to enhance the spatial resolution of typical fiber Bragg grating–based strain gauges has been experimentally verified in this study. Just analyzing the intensity of the reflection spectrum of a sampled fiber Bragg grating, its inner deformation profile has been obtained with a spatial resolution of Formula L

Tilted diffraction gratings inscribed by femtosecond laser have been manufactured within multimode optical fibers, located 60 μm below the fiber end-face. These structures have been characterized by studying their phase change distribution along the grating and their cross section. Based on the previous characterization, the angles of diffraction orders have been modeled with the beam propagation method in function of the tilt angle. Resulting far field patterns exhibit a good order filter performance with several applications such as wavelength demultiplexing or displacement sensing.

A new bonding sensor for composite material manufacturing based on a single uniform Fiber Bragg Grating (FBG) is proposed and demonstrated in this paper. The sensor design includes two edge sections attached to the bonded parts and a central free region, all made of an epoxy resin block of constant thickness, where the FBG is embedded. The quality of this bond is quantified by measuring the FBG spectral evolution. The strain transfer from the sensor to the embedded fiber allows to distinguish between two typical bonding defects in composite structures: asymmetric deformation and slide of bonded parts. Both cases have been experimentally checked and the results are in good agreement with simulations based on a simplified model using the Finite Elements Analysis (FEA) and Transfer Matrix methods, even under complex deformations.

In this paper a proof of concept of a switchable erbium doped fiber ring laser (EDFRL) system, demonstrated for temperature sensors multiplexing, was carried out showing a high linearity and a sensitivity of 1.55 GHz/°C. The system is based on the detection in the electrical domain of the resulting beat signal of two different laser lines generated by an EDFRL that operates in single longitudinal mode regime (SLM). The SLM operation is achieved by the use of phase shift fiber Bragg gratings (PSFBGs) with ultra-narrow bandwidths (BW) of 6 pm. Two Independent cavities for the reference signal and sensor signal generation are used to avoid the wavelength competition and to improve the stability. This configuration allows the control of the temperature operation point of the system by adjusting the temperature in the reference side. The authors are grateful to the Spanish government project TEC2010-20224-C02, to the Spanish Ministry of Education and Culture, and to the grant AP2009-1403.

In this work, an In-fiber Mach-Zehnder Interferometer has been manufactured within a standard optical fiber using direct inscription with a femtosecond laser. Different geometries have been tested in order to enhance the sensitivity of manufactured devices to high temperature sensing, reducing the total device up to 300µm. The achieved results exhibit a remarkably good response for temperature sensing up to 650°C. This work has been supported by the projects TEC2013-47264-C2-1-R, TEC2016-76021-C2-2-R and grant BES-2014-069736 of the Spanish government and by the Parliament of Cantabria postdoc grant.

Automatic discrimination between strain and temperature in a Brillouin optical time domain analyzer via artificial neural networks is proposed and discussed in this paper. Using a standard monomode optical fiber as the sensing element, the ability of the proposed solution to detect the known changes that the Brillouin gain spectrum exhibits depending on the applied temperature and/or strain will be studied. Experimental results, where different simultaneous strain and temperature situations have been considered, will show the feasibility of this technique. This work has been supported by the projects TEC2013-47264-C2-1-R and TEC2016-76021-C2-2-R.

In this paper, an In-fiber Mach-Zehnder interferometer inscribed by femtosecond laser for curvature sensing has been designed and manufactured. Its operating principle consists of a secondary waveguide inscription working as a sensing arm. This waveguide has been manufactured using asymmetric structures with an average refractive index change of 1.1 × 10-2 in its guiding section measured by refracted near-field profilometry. The overall arm/cladding index difference is higher than its core/cladding counterpart, which is a suggested condition for device operation following preliminary simulations. The manufactured Mach-Zehnder interferometer exhibits a linear response to bending radius that is also dependent on an established bending axis. Sensitivity has been measured up to 9.49 nm/m-1 for curvature ranges from 0 to 14 m-1. Simulation results using the beam propagation method and conformal mapping transformation to convert bending action into a tilt refractive index agree with experimental results for the same index parameters. Preliminary temperature test shows a remarkable cross sensitivity of 0.0024(3)m-1/-C up to 180 -C. This work has been supported by the pro-jects TEC2013-47264-C2-1-R, TEC2016-76021-C2-2-R (AEI/FEDER, UE) financed with FEDER funds, and grant BES-2014-069736 of the Spanish government and by the Parliament of Cantabria postdoc grant.

In this work, a fiber ring laser sensor has been employed to analyze the influence of passive Er-doped fiber acting as saturable absorber. Lasing modes of fiber ring lasers can be reduced by properly locating a saturable absorber that narrows the spectral width of the FBG that selects the lasing wavelength. Employing commercial Er-doped fiber, different configurations have been evaluated, reaching the SLM regime on a ring cavity of several meters. Thus, the achieved strain response of the whole sensor behaves at its mirror FBG, exhibiting linear response to strain. This work has been supported by the project TEC2013-47264-C2 and TEC2016-76021-C2, of the Spanish government and by a Parliament of Cantabria postdoc grant.

The spectral narrowing of Fiber Bragg Gratings (FBGs) introduced by unpumped Er-doped fiber (EDF) was analyzed for fiber lasers (FL). Owing to spatial hole burning (SHB), the spectral response of a virtual FBG can be employed for narrowing the band pass filter employed to determine the lasing wavelength of laser cavities. A common FL was mounted to analyze the spectral stability of the method, and a FL sensor for strain and temperature measurements was experimentally characterized to determine the stability of the narrowing effect achieved by the unpumped EDF, which acts as a virtual FBG. The results exhibited remarkably good narrowing effects of the spectral response of uniform FBGs. This research was funded by MINECO (Spain) project TEC 2016-76021-C2-R and FEDER funds.

We report a practical study of the thermal decay of cladding mode resonances in tilted fiber Bragg gratings, establishing an analogy with the “power law” evolution previously observed on uniform gratings. We examine how this process contributes to a great thermal stability, even improving it by means of a second cycle slightly increasing the annealing temperature. In addition, we show an improvement of the grating spectrum after annealing, with respect to the one just after inscription, which suggests the application of this method to be employed to improve saturation issues during the photo-inscription process. Á. González-Vila is supported by the F.R.S.-FNRS through a FRIA grant. C. Caucheteur is supported by the F.R.S.-FNRS. The authors would also like to thank the financial support from the ERC (European Research Council) Starting Independent Grant PROSPER (grant agreement No. 280161 – http://hosting.umons.ac.be/aspnet/erc-prosper/) and from the project TEC2013-47264-C2-1-R of the Spanish government.

In this work, a simple switchable dual-wavelength short-cavity fiber laser operating in a single-longitudinal-mode regime at room temperature and based on superposed fiber Bragg gratings is proposed and experimentally demonstrated. Only by introducing stress into one of the overwritten FBGs, single- or dual-wavelength laser oscillations can be attained. Either single- or multimode operation of the laser can be easily achieved. Single-longitudinal-mode emission has been verified in two different ways: the first corroboration has been conducted by the heterodyne detection of the output signal, and the second one has been carried out by using a high-resolution optical spectrum analyzer. This work was supported in part by the Spanish Comisión Interministerial de Ciencia y Tecnología within Project TEC2013-47264-C2-R and in part by FEDER funds.

After a mention of what it must be understood as Photonics field we will go into Optical Sensing area including the particular cases of the sensors using fiber technologies. Four significant cases will be presented and discussed

An ultra-long, low-threshold and high-stability Random distributed feedback fiber laser (RDF-FL) based on Erbium-doped fiber (EDF) to provide the gain medium, and single mode fiber (SMF) as a distributed mirror in combination with fiber-brag grating (FBG) to form the cavity is proposed in this paper. Typical random laser radiation for a SMF length of 50 km with a low-threshold of 10 dBm and high-stability (~0.1 dB) is achieved, thanks to the design of the cavity and the high-gain from the pumped erbium-doped fiber. Besides, 200 km quasi-losses random laser due to the distributed mirror and the FBG reflector is demonstrated. Financial support from the Spanish Comisión Interministerial de Ciencia y Tecnología within project TEC2013-47264- C2-1-R.

In this paper, the interference speckle patterns generated by multimode optical fibers are described. In our experience two types of interference are present, random interference between modes propagated in the fiber that give rise to speckle pattern, and not random speckle interference patterns using a Michelson interferometer generating a pattern of conventional interference. Multimode fibers using different materials and core radii have been obtained interference patterns quality characteristic reducing the effects of modal noise in fiber speckle patterns. Experimental results and their potential applications are presented. This work has been supported by the project TEC2013-47264-C2-1-R.

In this work, a frequency based processing scheme has been employed to improve the sensitivity of Fiber Specklegram Sensors (FSSs) applied to perimeter fences. Based on 1.8 km of multimode fiber, two laser sources with different spectral properties have been employed to feed the optical fiber. These two laser sources provoke sensing systems with very different SNRs to which the common frequency suppression method has been applied. The final results suggest enough improvement on SNR to reduce the quality of the laser source on perimeter sensors, also decreasing the final cost. This work has been supported by the project TEC2013-47264-C2-1-R.

A polarimetric Distributed Bragg Reflector (DBR) laser sensor in a low birefringent Er-doped fiber has been proposed. The spectral overlap of two uniform fiber Bragg gratings (FBG) has been employed as filtering technique to achieve a Single Longitudinal Mode (SLM) regime. By measuring the RF beat frequency between the two orthogonal polarized lasing modes and the absolute wavelength of one mode, both strain and temperature has been determined simultaneously.

A processing scheme able to obtain any arbitrary intragrating strain distribution of a fiber Bragg grating (FBG) is proposed and demonstrated. The processing method employs just the intensity of the FBG reflection spectrum to obtain its deformation profile by combining a geometrical processing scheme with the particle swarm optimization technique. The technique has been evaluated using several spectra generated from very heterogeneous strain distributions and with a real spectrum obtained from a 5 mm length FBG under an asymmetric perturbation. The achieved results exhibit a good spatial resolution (Δ≈0.5 mm) and repeatability. This work has been supported by the project TEC2010-20224-C02-02 and grant AP2009-1403

In this paper, a 50 Km remote and switchable fiber optic sensing system based on an erbium doped fiber ring laser is proposed. The system allows the integration of different interrogation methods based on the detection of wavelength or amplitude variations of the laser. This integration is carried out by sequentially activating different ring cavities by using an optical switch. With this scheme, the maximum gain zone of the erbium doped fiber amplifier can be used to increase the distance of the sensor system. A bending intensity sensor and a FBG based strain sensor were successfully tested for high and low-strain measurements respectively. This system includes all the active elements inside the monitoring station, enabling the remote monitoring of the sensors. The authors are grateful to the Spanish government project TEC2010-20224-C02, to the Spanish Ministry of Education and Culture, and to the grant AP2009-1403.

A narrow filtering technique based on the spectral overlapping of two uniform FBGs and applied to obtain a Single Longitudinal Mode (SLM) laser is proposed and demonstrated in this work. The two FBGs are spectrally detuned to reduce their coincident reflection response narrowing the equivalent filter bandwidth. A proof-of-concept linear laser has been built and tested exhibiting SLM operation even with temperature and strain variations. The authors are grateful to the Spanish government project TEC2010-20224-C02, to the Spanish Ministry of Education and Culture, and to the grant AP2009-1403.

Several easy-to-manufacture designs based on a pair of Fiber Bragg Gratings structure embedded in Carbon Fiber Reinforced Plastic (CFRP) have been explored. These smart composites can be used for strain and temperature discrimination. A Finite Elements Analysis and Matlab software were used to study the mechanical responses and its optical behaviors. The results exhibited different sensitivity and using a matrix method it is possible to compensate the thermal drift in a real application keeping a simple manufacture process.

Chemical analysis of dangerous materials entails a safety issue for the researchers. Laser Induced Breakdown Spectroscopy offers the possibility to analyze these materials away from them using Stand-Off Set-ups. To optimize the plasma induction, the remote focalization of the laser beam is of paramount importance. A custom Fiber Bragg Grating sensor system able to correct the laser beam focalization errors is proposed and experimentally checked. The optical transducer architecture and the preliminary obtained results are reported in this paper. This work has been co-supported by the Spanish Government through the project TEC2010-20224-C02- 02 and the grant AP2007-02230.

A fluid flow speed sensor based on Fiber Bragg Gratings (FBGs) is proposed and demonstrated. Two FBGs are attached to an arc-shaped plastic piece at the same distance from both edges. The fluid flow to be measured pushes the structure stretching one side and compressing the other one. The sensor has been successfully checked by subjecting the device, to different air flow controlled conditions inside of a pipe. Low energy impact detection has been also reported with the proposed device. This work has been supported by the project TEC2010-20224-C02-02 and grant AP2009-1403.

The distribution of the intensity of each speckle depends on the relative phases of modes in the multimode fiber, so they are extremely sensitive to external perturbations of the fiber. These perturbations can locally appear in the fiber without disrupting the entire optical assembly. If the refractive index of the outside medium of the fiber is changed, it can cause variations in the speckle pattern at the fiber output. Thus, by changing the refractive index of the outside medium the speckle pattern at the output fiber varies and its influence can be observed. In this paper we demonstrate the influence of the refractive index of liquids in the speckle pattern obtained by a multimode fiber. In order to obtain greater sensitivity of the experimental measurement, the fiber is bent in a U-shape and immersed in a liquid. The core and cladding are 240 microns and 250 microns, respectively. The intensity speckle field is then captured by a CCD camera in digital image format and processed by the computer with a Matlab program. The portion of fiber exposed to the disturbance of the liquid is located 2 meters before the exit of the fiber. The portion of the fiber in contact with the liquid is curved with a radius of 2 mm. The authors acknowledge the Spanish government for supporting this work through the TEC2010-20224-CO2-02 projects and with the grant AP2009-1403.

A novel all-dielectric metasurface in silicon-on-insulator technology made of arrayed circular slots is proposed and theoretically studied. The metasurface exhibits quasi-bound states in the continuum resonances, having an ultra-high Q-factor up to 5x108.

A method of measuring the refractive index of liquid based on the change of speckle contrast in fiber is presented. The correlation of propagation losses, proportional to the speckle contrast, is used when a section of U-shaped fiber without cladding is submerged in the liquid. The proposed method is easy to perform, simple and low cost, allows quick measurements, with good accuracy and in a large dynamic range.

Experimental results from interference between the speckle patterns generated by multimode fibers are presented. This scheme can increase the multiplexing capability of fiber specklegram sensors by encoding extra perturbations within the fringes resulting from the Michelson interferometer.